The electronics industry is striving to further miniaturize semiconductors and integrated circuits while increasing design complexity. To accomplish that result, the individual, active electrical devices such as transistors, diodes and the like used to create the circuitry and the interconnects between such devices must be fabricated on an increasingly small scale. As circuitry dimensions decrease, contaminants present during the fabrication of electronic devices cause significantly more device failures and malfunctions. Therefore, surface contaminants must be carefully removed from the circuit assembly to maintain quality standards and to maximize the yield of fully functioning integrated circuits.
Contaminants present during integrated circuit fabrication include photoresist materials, residual organic and metallic contaminants such as alkali metals and native/metallic oxides. Metallic films comprising metal oxides and metal halides can also be inadvertently deposited onto electronic devices during immersion into etchant or resist stripper baths, both which may contain metal ions and free metals in solution. Likewise, corrosive chlorides are deposited onto such assemblies through various handling operations and plasma etching processes. These contaminants can weaken or embrittle the electrical connectors of the device and can cause the layers of the device to delaminate resulting in current leakages or physical failure. Sodium-containing contaminants are particularly problematic because sodium can readily diffuse into the silicon matrix causing device failure.
Chemical cleaning agents are typically used to remove bonded or adsorbed metallic oxides and corrosive chloride residues from water surfaces between the numerous individual steps required to fabricate an integrated circuit. Conventional chemical cleaning processes are typically performed using a series of acid and rinse baths. These cleaning processes are often characterized as "wet" techniques because the electronic device is immersed in a liquid cleaning solution immediately prior to performing the next processing step. Numerous problems are encountered when using liquid cleaning solutions including incomplete removal of the cleaning agent from the surface and new contaminants being introduced onto the surface to be cleaned. Hazardous liquid wastes generated during the process must also be disposed.
A typical wet cleaning process for removing film contaminants from a bare silicon or thermally grown silicon oxide crystal comprises immersing the wafer into an inorganic stripper such as sulfuric acid/hydrogen peroxide followed by immersing in a sulfuric acid/oxidant mixture and rinsing with deionized water; immersing the wafer into a mixture of water/ammonium hydroxide/hydrogen peroxide to remove metal oxides and metals followed by rinsing with deionized water; immersing the still wet wafers into a mixture of water/hydrochloric acid/hydrogen peroxide to desorb atomic and ionic contaminants; and rinsing the cleaned wafers with distilled water and drying the same in a inert atmosphere such as nitrogen.
Numerous problems are associated with conventional "wet" cleaning processes. For example, ammonia and HCl vapors can mix to form particulate smoke containing colloidal ammonium chloride particles which can contaminate the wafer. Special care must also be taken to prevent hydrogen peroxide from being depleted from the cleaning solution because ammonium hydroxide in the absence of hydrogen peroxide acts as a silicon etchant. Additional contaminants may also be introduced into the system during the numerous distilled water rinsing steps required to remove cleaning residue. Finally, trace moisture must be removed, typically via high temperature vacuum applications, before conducting the next processing step.
Disadvantages associated with wet wafer cleaning processes have led to a search for "dry" cleaning processes wherein the cleaning agent is applied and removed in the vapor state. In order to conduct a vapor phase cleaning process, the products formed by contacting the cleaning agent and the contaminants must possess sufficient volatility to enable essentially complete removal from the surface to be cleaned. Manufacturers are continually searching for "dry" cleaning agents and vapor-phase processes for using such cleaning agents which eliminate the enumerated problems wherein high quality electronic devices can be fabricated without using environmentally harmful reagents.
T. Ito and coworkers, Proc. 2nd Int'l Symp. on Cleaning Technol. In Semicond. Dev. Manuf., 92-12 72 [1992] disclose a process for cleaning iron and aluminum contaminants from a silicon surface wherein photoexcited chlorine radicals are employed. Both iron and aluminum on the silicon surface are completely removed by UV-excited dry cleaning at a cleaning temperature of 170.degree. C. Iron and aluminum concentrations on the silicon surface were decreased two orders of magnitude when the surface was etched only 2 nm deep. Ito and coworkers have also reported that the photoexcited chlorine radicals decrease the amount of sodium residing on the substrate's surface. Applicants believe that the sodium chloride formed during the process can diffuse into the bulk of the substrate causing deleterious electrical effects.
Many of these disadvantages have been overcome by U.S. Pat. No. 5,094,701, assigned to Air Products and Chemicals, Inc., which discloses a residue-free cleaning process for removing metal-containing comtaminants from a surface of a substrate of the type used in manufacturing semiconductor devices. The process comprises contacting the substrate with an effective amount of a beta-diketone or beta-ketoimine dispersed in an atmosphere capable of oxidizing the metal-containing contaminants at a temperature sufficient to form volatile metal-ligand complexes on the surface of the substrate. The volatile metal-ligand complexes are sublimed from the surface of the substrate leaving essentially no residue. The process is particularly effective in removing copper-, iron- and silver-containing contaminants from substrates of the type used in the electronics industry.
The electronics industry is seeking an improved process for removing metal-containing contaminants, and particularly alkali-metal containing contaminants such as the oxides, hydroxides and salts of sodium, from the surface of substrates such as silicon which are commonly used in fabricating electronic devices.